The Holocene dilemma

Once regarded as a long period of climate stability, it is now clear from marine and terrestrial records that the entire 12,000 yr of the Holocene is punctuated by a series of robust cycles (not necessarily periodic) of millennial duration. Many of those records demonstrate that the cycle pacing occurs within a band of 1000 to 2000yr. Curiously though, ice-core records reveal few climate shifts within the Holocene, the main events occurring at about 8200 yr and within the past 2000 yr where the most recent cycle corresponds to the Medieval Warm Period-Little Ice Age oscillation.

Upon extending their analyses of the two petrological tracers (haematite-stained grains and Icelandic glass) into the Holocene, Bond et al. (1997, 1999, 2001) made an unexpected discovery. They found that the robust variations of the last glacial extended through the entire Holocene with a pacing that fell within the 1470 ± 500-yr pacing of the glacial cycles (Fig. 24.6). Given the absence of large glaciers and ice sheets during most of the Holocene, the mechanism for the persistent and simultaneous increases in both tracers within the range of the 1470 ± 500-yr could not have been synchronous increases in iceberg discharge from more than one source as previously thought. Bond et al. (1999) concluded that it was more likely that recurring southward advections of cooler surface waters from the seas north of Iceland carried ice with IRD rich in both haematite-stained grains and Icelandic glass, implying that the cycle reflected a mechanism operating within the ocean-atmosphere system. Given the similarity in composition and pacing of the Holocene and glacial petrological cycles, it seemed possible that the glacial petrological cycles might be a manifestation of the same climate mechanism that operated during the Holocene.

Hence, the petrological record of the 1470-yr cycle suggested that whatever its fundamental origin, the cycle gave rise to oscillations in the ocean-atmosphere system that controlled deposition of at least some of the IRD associated with D-O cycles in the North Atlantic. That conclusion would seem to lend further support to the climate-centred hypothesis for glacier-ocean interactions. The argument has been criticized, however, because the Holocene cycles do not have the precise 1470-yr periodicity of the D-O oscillations (e.g. Schulz et al., 2002; Fig. 24.6).

Braun et al. (2004, 2005) have proposed an intriguing solution to this problem. Building on evidence in Bond et al. (2001) that the Holocene cycles may have been forced by variations in solar irradiance, Braun and his colleagues made the remarkable discovery that their climate system model CLIMBER-2, when forced by the DeVries solar cycle (210yr) and the Gleissberg solar cycle (88yr), can produce robust D-O events with an exact period of 1470 yr under glacial boundary (Fig. 24.7) conditions but not under Holocene boundary conditions. Their surprising results have far reaching implications for the fundamental mechanisms that underlie abrupt climate change, and they clearly deserve much further investigation and rigorous testing.

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